Method for computer-aided signaling in an automatic repeat request procedure

ABSTRACT

In an automatic repeat request procedure (ARQ), a first message is transmitted from a first computer unit to a second computer unit. Upon detection of a transmission error in the first message, a back-signalling message is coded in the second computer unit and transmitted to the first computer unit. The back-signalling message has at least one request field having the length of exactly one bit. A second message is requested by means of the request field. The second message contains additional information required for the error correction of the first message, or is the repetition of a message already transmitted or lost. The second message is transmitted to the second computer unit and decoded in the second computer unit together with the first message.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for computer-aided backsignaling in an automatic repeat request procedure wherein a requestedfurther message which contains redundancy information for the purpose oferror correction of the first message is identified uniquely from theback-signaling message.

2. Description of the Prior Art

In a so-called type II automatic repeat request procedure (ARQ-IIprocedure), messages are exchanged between two computer units. In thiscase, an information word to be transmitted is coded by using aperforation code. An information word is understood in this context as abit sequence of arbitrary length which contains the actual informationto be transmitted.

Different variants of perforation codes are known, for example there isthe so-called perforation convolutional code, which is described indocument [1]. Also described in document [1] is the automatic Type IIrepeat request procedure.

Further codes suitable for ARQ-II are known, for example, from document[2]. The Type II automatic repeat request procedure also can be used forthis type of perforation code.

During coding, messages are formed which, on the one hand, contain theinformation of the information word and, on the other hand, haveredundancy information for error correction, independent of the specificperforation code employed, such messages are formed for the informationword in a way which is characteristic of the respective perforationcode.

In the ARQ procedure, the messages are transmitted in such a way that,at the start, a first message is transmitted from a first computer unitto a second computer unit. The first message contains at least theinformation of the information word and information for the purpose oferror detection. If, during decoding of the first message, an error isdetected in the second computer unit, a second message is requested fromthe first computer unit by means of a back-signalling message whichcontains redundancy information for the purpose of error correction ofthe first message. The second message is also coded during formation ofthe messages. The second message is transmitted to the second computerunit. The second message is decoded in the second computer unit togetherwith the first message. For this purpose, the first message must bebuffered in the second computer unit in coded form. If the first messageand the second message are decoded together, it is then possible tocorrect errors with an increased probability by means of the redundancyinformation contained in the second message relating to the firstmessage. If decoding without errors still remains impossible, furtherredundancy in the form of further messages from the first computer unitis requested. After the transmission of a third message, which hasfurther redundancy information relating to the first message, to thesecond computer unit, the third message is decoded in the third computerunit together with the first message and the second message. Thisprinciple can be continued with further messages which contain furtherredundancy information relating to the first message until decodingwithout error is possible in the second computer unit. This mode ofprocedure is described in document [1].

In the back-signalling message, by means of which a further message and,thus, further redundancy information is requested, it is necessary tospecify which further message is, requested, that is to it must be clearfrom the back-signalling message for the first computer unit whether thesecond message, the third message, etc. is to be transmitted.Furthermore, the information on identifying the further message is ofgreat importance, since in order to correctly decode the composedmessage, which is produced, for example, from the first message and thesecond message, it is necessary to know which messages are beingcombined. This purpose is fulfilled by the identification bit in theback-signalling message, whose function also can be understood as acounter.

The problem to which the present invention is directed is to specify anARQ procedure in which a requested further message which containsredundancy information for the purpose of error correction of the firstmessage is identified uniquely from the back-signalling message.

SUMMARY OF THE INVENTION

Accordingly, pursuant to a method of the present invention, the ARQprocedure is carried out by means of a back-signalling message whichcontains exactly one bit for the purpose of identifying the requestedfurther message by this means, the redundancy information relating tothe first message is transmitted to the second computer unit in order torender error correction possible; that is, decoding without errors ofthe first message together with the further transmitted message.

Specifying the requested message in exactly one bit creates a verysimple possibility of identifying the requested message in the firstcomputer unit. Furthermore, the use of only one bit for the purpose ofidentifying the requested message minimizes the transmission rate, whichis required for transmitting the back-signalling message, with respectto the request for a further message.

The evaluation of the back-signalling message in the first computer unitis also performed very simply, since only one bit needs to be evaluatedin order to obtain the information as to which message is to betransmitted to the second computer unit. As a result, the arithmeticcapability required by a computer for evaluation purposes is kept verylow, since there is no need to carry out any complicated evaluationprocedures.

Additional features and advantages of the present invention aredescribed in, and will be apparent from, the Detailed Description of thePreferred Embodiments and the Drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a flowchart in which the course of a typical automaticrepeat request procedure is represented in the form of a messageflowchart.

FIG. 2 shows a flowchart in which a possible structure of theback-signalling message is represented, and

FIGS. 3a to 3 d show message flowcharts in which different instances oferror in the ARQ procedure are described.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 represents, in the form of a message flowchart, the automaticrepeat request procedure (Type II, ARQ-II Automatic Repeat RequestProcedure) of so-called Type II which is described, for example, indocument [1].

Messages are exchanged between two computer units in the ARQ-IIprocedure. In this case, an information word I to be transmitted iscoded 101 in a first computer unit C1 by using a perforation code.Furthermore, in addition to the information word I, a bit sequence CRCis provided for the purpose of error detection, for example by means ofa CRC procedure (Cyclic Redundancy Check) and/or with a prescribednumber of tailbits T, the number of which corresponds to the degree of afeedback shift register used for coding. The tailbits serve in theirtrue function to bring the feedback shift register into a defined finalstate; for example, into a state in which all registers of the feedbackshift register have the value 0.

The information word I is coded 101; for example, using a perforationconvolutional code or a code which is described in Document [2]. Thisleads to an intermediate word ZW which is “perforated” in a furtherstep. Under a perforation is a selection, dependent on a perforationcode employed and prescribed by the perforation code, of bits of theintermediate word ZW, and the partitioning of the intermediate word ZWinto messages to be transmitted. In this case, a first message N1 isformed which contains at least the information word I and informationwhich is used to detect transmission errors in the decoding of the firstmessage N1 in a second computer unit C2. In addition, further messagesNi are formed from the intermediate word ZW and stored in the firstcomputer unit C1. The index i is a unique indication of the respectivefurther message Ni. and i is an arbitrary natural number. For example,the index i is used with the value 2 to denote a second message N2 andis used with the value 3 to denote a third message N3. The furthermessages Ni respectively contain redundancy information relating to thefirst message N1 or to further messages Ni, which is used together withthe previously transmitted messages N1 to Ni-1 in the second computerunit C2 for the purpose of decoding the information word I withcorrection of errors. Decoding with correction of errors is understoodin this context as the following mode of procedure after thetransmission of the further messages Ni to the second computer unit C2.Usually, in each case a further message Ni is transmitted to the secondcomputer unit C2, wherein the further message Ni is decoded togetherwith non-decoded, previously received messages buffered in the secondcomputer unit C2.

After transmission of the first message N1 from the first computer unitC1 to the second computer unit C2 102, the first message N1 is received103 in the second computer unit C2 and decoded 104 also an errordetection procedure is applied 105 to the decoded first message N1; forexample, with the aid of the additional bit sequence CRC and/or with theaid of the tailbits T. If the first message N1 was transmitted withouterrors, the information word I obtained by decoding can be processed inthe second computer unit.

If, however, an error is detected, for example a transmission errorwhich cannot be remedied with the aid of an error correction procedureprovided optionally in addition thereto, an item of redundancyinformation relating to the first message N1 is required in theperforation code. The required redundancy information is contained inthe further message Ni. In order to obtain this redundancy information,a back-signalling message R is coded 106 in the second computer unit C2and transmitted 107 to the first computer unit C1. A further message Niis requested by means of the back-signalling message R. Theback-signalling message R is received 108, decoded 109 and evaluated bythe first computer unit C1.

A possible structure of the back-signalling message R is represented inFIG. 2. The back-signalling message R has at least the followinginformation:

a backward information field PT in which it is specified whetheradditional information was at all requested by means of thebackward-signalling message R; the backward information field PT can,for example, be realized by one bit indicated by means of a first bitvalue that a further message Ni is being requested;

a sequential number field SN which contains a counter, for example ofwidth 5 bits, in order to cover a number range of 0 to 31; thesequential number field SN uniquely characterizes the membership of theback-signalling message R to a specific information word I;

a request field ID with a specification of the requested furthermessage; according to the present invention, this field of theback-signalling message R has a length of exactly one bit.

Additional information fields can, of course, be present in theback-signalling message R in a way specified by protocol. Again, it isprovided in one variant that the back-signalling message R has nobackward information field PT.

If, on account of an error occurring in the transmission of the firstmessage N1, the second message is requested by means of theback-signalling message R, the field of the specification of therequested further message ID has a first value. If a back-signallingmessage R is received by the second computer unit C2 after thetransmission of the first message N1 from the first computer unit C1,and if the request field ID has the first value, the first computer unitC1 can detect that the second message N2 is being requested.

On the basis of the back-signalling message R with the request fieldwith the first value, the second message N2 with additional redundancyinformation relative to the first message N1 is transmitted 110 from thefirst computer unit C1 to the second computer unit C2. The secondmessage N2 is received 111 and stored in the second computer unit C2.The coded first message N1, which was stored in coded form in the secondcomputer unit C2, is decoded 112 together with the second message N2. Anerror detection procedure is applied, in turn, to the decoded messagethereby formed.

If, in turn, an error is detected, a further back-signalling message Ris coded and transmitted to the first computer unit C1. The thirdmessage N3 is requested in this back-signalling message R, and this isspecified by a second value of the request field ID. The back-signallingmessage R is received in the first computer unit C1, and the requestfield ID is evaluated. It is detected in the first computer unit C1 bymeans of the second value in the request field ID that the third messageN3 is being requested. The third message N3 is transmitted to the secondcomputer unit C2 on the basis of this value in the request field ID.

After the reception of the third message N3 in the second computer unitC2, the coded first message N1, the coded second message N2 and thethird message N3 are decoded together. An error detection procedure isapplied, in turn, to the decoded message formed therefrom.

If an error is detected, a further back-signalling message R is codedand transmitted to the first computer unit C1. A fourth message N4 isrequested in this back-signalling message R and is specified by thefirst value of the request field ID.

This mode of procedure can be repeated iteratively until it is possibleto decode the received messages in the second computer unit C2 withouterrors. In each iteration step, a further message Ni is respectivelyrequested, which is characterized by a request field ID which changesthe value in each case with each iteration step.

If the decoding of the combined messages can be realized without errors,something for which the probability increases with a larger amount ofadditional redundancy information, that is to say with a larger numberof further messages Ni, the message decoded without errors contains theinformation word I, which is then processed.

Two possible embodiments of the method of the present invention aresketched in FIGS. 3a and 3 b. The lightning symbol in the respectivetransmission arrows of the individual messages symbolizes the occurrenceof an error in the transmission of the corresponding message.

In FIG. 3a, an error occurs in the transmission of the first message N1.This fact is detected in the second computer unit C2, and theback-signalling message R is transmitted to the first computer unit C1.The request field ID of the back-signalling message R has the firstvalue, for example the value 0. Thereupon, the second message N2 istransmitted to the second computer unit C2.

In FIG. 3b, transmission errors occur both in the first message N1 andin the transmission of the second message N2. In this case, the secondmessage N2 is requested by means of a back-signalling message R with thefirst value in the request field ID. The transmission of the secondmessage N2 is so strongly disturbed in this case, which is representedin FIG. 3b, that the second message N2 does not arrive at the secondcomputer unit C2.

In accordance with a time counter T, it is decided in the secondcomputer unit C2 to request once again the requested, but not received amessage N2. For this purpose it transmits a further back-signallingmessage R, once again with the first value in the request field ID. Thefirst computer unit C1 will transmit the message N2 again afterreceiving this message.

This example illustrates the necessity of the counter in the requestfield ID. Specifically, if the third message N3 were transmitted anywayfrom the first computer unit C1 upon request, the third message N3 couldnot be correctly assigned in the second computer unit C2 and decodingwould be impossible.

However, exactly one bit in the request field ID suffices to identifyuniquely the respectively requested message, since it only needs to bemade known in the first computer unit C1 whether the messagerespectively already transmitted has to be repeated or whether the nextmessage present is to be sent.

Although the present invention has been described with reference tospecific embodiments, those of skill in the art will recognize thatchanges may be made thereto without departing from the spirit and scopeof the invention as set forth in the hereafter appended claims.

The following publications have been quoted in this document:

[1] J. Hagenauer, IEEE Transactions on Communications, Vol. 36, No. 4,Rate-Compatible Punctured Convolutional Codes (RCPC-Codes) and theirApplications, S. 389 to 400, April 1988

[2] J. Serrat-Fernández and J. Dalmau-Royo, Hybrid-ARQ System forHF-Channels Based on Codeword Partitioning, IEE Proc. Commun., Vol. 143,No. 2, S. 73-77, April 1996

What is claimed is:
 1. A method for computer-aided back signaling in anautomatic repeat request procedure, comprising the steps of: coding aninformation word, which has an arbitrary number of bits, in a firstcomputer unit into at least a first message and further messages byusing a perforation code; transmitting the first message in the firstcomputer unit to a second computer unit; receiving the first message inthe second computer unit; decoding the first message in the secondcomputer unit; checking in the second computer unit whether the firstmessage was transmitted without errors; coding and transmitting at leastone back-signaling message to the first computer unit for the case inwhich the first message was not transmitted without errors, wherein theback-signaling message has at least one request field with a length ofexactly one bit by means of which a second message is identified;receiving and decoding the back-signaling message in the first computerunit; transmitting the requested second message to the second computerunit, wherein the requested second message contains an additional itemof information relating to the first message for the purpose of errorcorrection together with the first message in the second computer unit;and receiving and decoding the second message in the second computerunit together with the first message; wherein a value of the at leastone request field does not change upon further iterative transmissionsof further back-signaling messages upon further messages being receivedin the second computer unit with errors.
 2. A method as claimed in claim1, wherein the perforation code is a perforation convolutional code. 3.A method as claimed in claim 1, further comprising the steps of:requesting further messages by means of further back-signaling messages;transmitting the further messages from the first computer unit to thesecond computer unit; and decoding the further messages in the secondcomputer unit together with the previously received, coded messages. 4.A method as claimed in claim 2, further comprising the steps of:requesting further messages by means of further back-signaling messages;transmitting the further messages from the first computer unit to thesecond computer unit; and decoding the further messages in the secondcomputer unit together with the previously received, coded messages.